In recent years, mobile devices such as cell-phones (and in particular smart-phones), tablets and laptops have become ubiquitous. Many of these devices include one or two compact cameras including, for example, a main rear-facing camera (i.e. a camera on the back face of the device, facing away from the user and often used for casual photography), and a secondary front-facing camera (i.e. a camera located on the front face of the device and often used for video conferencing).
Although relatively compact in nature, the design of most of these cameras is similar to the traditional structure of a digital still camera, i.e. it comprises a lens module (or a train of several optical elements) placed on top of an image sensor. The lens module refracts the incoming light rays and bends them to create an image of a scene on the sensor. The dimensions of these cameras are largely determined by the size of the sensor and by the height of the optics. These are usually tied together through the focal length (“f”) of the lens and its field of view (FOV) a lens that has to image a certain FOV on a sensor of a certain size has a specific focal length. Keeping the FOV constant, the larger the sensor dimensions (e.g. in a X-Y plane), the larger the focal length and the optics height.
In recent times, a “folded camera module” structure has been suggested to reduce the height of a compact camera. In the folded camera module structure, an optical path folding element (referred to hereinafter as “OPFE”) e.g. a prism or a mirror (otherwise referred to herein collectively as a “reflecting element”) is added in order to tilt the light propagation direction from perpendicular to the smart-phone back surface to parallel to the smart-phone back surface. If the folded camera module is part of a dual-aperture camera, this provides a folded optical path through one lens module (e.g. a Tele lens). Such a camera is referred to herein as a “folded-lens dual-aperture camera” or a “dual-aperture camera with a folded lens”. In general, the folded camera module may be included in a multi-aperture camera, e.g. together with two “non-folded” camera modules in a triple-aperture camera.
In addition to the lens module and sensor, modern cameras usually further include a mechanical motion (actuation) mechanism for two main purposes: focusing of the image on the sensor, and optical image stabilization (OIS). For focusing, in more advanced cameras, the position of the lens module (or at least of a lens element in the lens module) can be changed by means of an actuator and the focus distance can be changed in accordance with the captured object or scene.
The trend in digital still cameras is to increase the zooming capabilities (e.g. to 5×, 10× or more) and, in cell-phone (and particularly smart-phone) cameras, to decrease the sensor pixel size and to increase the pixel count. These trends result in greater sensitivity to camera shake for two reasons: 1) greater resolution, and 2) longer exposure time due to smaller sensor pixels. An OIS mechanism is required to mitigate this effect.
In OIS-enabled cameras, the lens module lateral position can be moved, or the entire camera module can be tilted in a fast manner to cancel camera shake during-image capture. Camera shakes shift the camera module in 6 degrees of freedom, namely linear movements in X-Y-Z, roll (“tilt about” or “tilt around”) the X axis, yaw (tilt around the Z axis) and pitch (tilt around the Y axis). While the linear motion in X-Y-Z negligibly affects the image quality and does not have to be compensated, compensation of the tilt angles is required. OIS systems shown in known designs (see e.g. US 20140327965A1) correct yaw and pitch, but not roll motion.
A folded-lens dual-aperture camera with an auto-focus (AF) mechanism is disclosed in Applicant's US published patent application US 20160044247, the description and figures of which are incorporated herein by reference in their entirety.